As costs of energy increase along with concerns about global warming due to consumption of fossil fuels to generate energy, there is an every increasing need for more efficient lighting technologies. These demands, coupled with rapid improvements in semiconductors and related manufacturing technologies, are driving a trend in the lighting industry toward the use of light emitting diodes (LEDs) or other solid state light sources to produce light for general lighting applications, as replacements for incandescent lighting and eventually as replacements for other older less efficient light sources.
The actual solid state light sources, however, produce light of specific limited spectral characteristics. To obtain white light of a desired characteristic and/or other desirable light colors, lighting devices based on solid state sources have typically used sources that produce light of two or more different colors or wavelengths. One technique involves mixing or combining individual light from LEDs of three or more different wavelengths (single or “primary” colors), for example from Red, Green and Blue LEDs. Another approach combines a white LED source, which tends to produce a cool bluish light, with one or more LEDs of specific wavelength(s) such as red and/or yellow chosen to shift a combined light output to a more desirable color temperature. Adjustment of the LED outputs offers control of intensity as well as the overall color output, e.g. color and/or color temperature of white light.
To provide efficient mixing of the various colors of the light and a pleasing uniform light output, Advanced Optical Technologies, LLC (AOT) of Herndon, Va. has developed a variety of light fixture configurations that utilize a diffusely reflective optical integrating cavity to process and combine the light from a number of solid state sources. By way of example, a variety of structures for AOT's lighting systems using optical integrating cavities are described in US Patent Application Publications 2007/0138978, 2007/0051883 and 2007/0045524, the disclosures of which are incorporated herein entirely by reference.
In recent years, techniques have also been developed to shift or enhance the characteristics of light generated by solid state sources using phosphors, including for generating white light using LEDs. Phosphor based techniques for generating white light from LEDs, currently favored by LED manufacturers, include UV or Blue LED pumped with phosphors and quantum dots pumped with UV LEDs.
There are a variety of structures and techniques that use phosphor to enhance the characteristics of the LED light output, although such techniques typically operate in one of two ways, as summarized below. In a UV LED that pumps RGB phosphors or quantum dots, non-visible UV light excites the mixture of red-green-blue phosphors or dots to emit light across the visible spectrum. There is no direct contribution of visible light from the UV LED semiconductor chip. In the other typical approach, a Blue LED is pumped with one or more phosphors or dots within the LED package. Some of the blue light from a blue LED chip (460 nm) excites the phosphor or dots to emit yellow light and then the rest of the blue light is mixed with the yellow to make white light, albeit of cool bluish character. Additional phosphors or dots can be used to improve the spectral characteristics. In either case, the phosphor or quantum dots material typically has been integrated directly into the LED and/or its package, for example by doping a portion of the package or by coating the portion of the package through which the light emerges. Phosphors have also been used on reflectors or transmissive layers inside of the package containing the actual LED chip.
AOT has also proposed to utilize phosphors, including quantum dot phosphors, on macro-scale components of their cavity based fixture optics. Their U.S. Pat. No. 7,144,131, the disclosure of which is incorporated herein entirely by reference, for example proposed improvements to semiconductor-based systems for generating white light, by integrating the phosphor into a reflective material of an external structure. In a disclosed example using an optical integrating cavity for lighting applications, one or more solid state energy source packages (typically LEDs) emit light energy of a first wavelength. In the cavity example, the cavity comprises a diffuse reflector outside the LED package(s) that has a diffusely reflective surface arranged to receive light energy from the source(s). At least some of that light energy of the first wavelength excites one or more phosphors or dots doped within the cavity reflector to emit visible light, including visible light energy of at least one second wavelength different from the first wavelength. Visible light emitted by the phosphor or dots is reflected by the diffuse surface of the reflector, and thereby integrated in the cavity. The integrated light may include some visible light from the solid state source(s). The optical aperture of the reflector/cavity (and possibly one or more additional downstream optical processing elements) directs the integrated light, including light from the phosphors or quantum dots, to facilitate the particular general lighting application.
As noted above, the phosphors used in solid state lighting may include quantum dots, sometimes referred to as nano phosphors or nano crystals or as quantum dot phosphors. Quantum dots are nano scale semiconductor particles, typically crystalline in nature, which absorb light of one wavelength and re-emit light at a different wavelength, much like conventional phosphors. However, unlike conventional phosphors, optical properties of the quantum dots can be tailored as a function of the size of the dots. In this way, for example, it is possible to adjust the absorption spectrum and/or the emission spectrum of the quantum dots by controlling crystal formation during the manufacturing process so as to change the size of the quantum dots. Thus, quantum dots of the same material, but with different sizes, can absorb and/or emit light of different colors. For at least some exemplary quantum dot materials, the larger the dots, the redder the spectrum of re-emitted light; whereas smaller dots produce a bluer spectrum of re-emitted light. Performance of some quantum dots may be tailored by other means. These unique characteristics make quantum dots particularly attractive for solid state lighting where a specifically tailored color shift of some of the light may be desired, in order to provide a desired spectral characteristic in white light or to otherwise shift the color of the light output produced from limited numbers of wavelengths in the light from the solid state sources, including for general lighting applications.
As typically utilized in various lighting applications, quantum dots are confined in some form of solid structure, e.g. as a paint or solid surface coating or otherwise doped into a material of a substrate. In such a solid matrix, the efficiency of quantum dot materials remains relatively low, e.g. around 30% or less. Use of such inefficient materials in general lighting applications reduces the benefits otherwise obtained by use of a solid state light emitter as the light source.
Hence a need exists for a technique to improve efficiency of operations of quantum dot materials in general lighting applications. It is known that quantum dots in liquids exhibit higher efficiencies than in solids, however, there has been no suggestion of a practical way to utilize quantum dots in a liquid in the context of a solid state lighting device, particularly one adapted for a general lighting application.